When downloading and installing games using Mac OS X 10.12/10.13 (Sierra), there is a slightly different process.
Instead of double clicking to install games, you must instead drag the icon into your Applications Folder. See the steps below:
1. Open Finder from your computer and then click on Applications
2. When instructed to double click to install the game, ignore this and instead drag the icon into Applications and open the game from there.
3. This will ensure your game is properly installed and can be found again when you wish to play it next time.